Electric machine cooling systems and methods

ABSTRACT

An electric machine including a shaft, a rotor back assembly surrounding a portion of the shaft, and two or more permanent magnets radially positioned around the perimeter of the rotor back assembly. The electric machine also includes a rotor fan with multiple fan blades formed in an exterior surface of the rotor back assembly and one or more ventilation channels extending through the rotor back assembly. Methods of exporting heat from an electric machine, wither from a machine housing or through the shaft is also disclosed. The heat exportation methods feature the circulation of a fluid with the rotor fan through the ventilation channels and into contact with the housing, or exporting heat from the rotor back assembly through the shaft.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/336,484 filed on Jun. 2, 2021, entitled “Electric Machine RotorCooling Systems and Methods”, which is a continuation of U.S.application Ser. No. 17/082,930 filed on Oct. 28, 2020, entitled“Electric Machine Rotor Cooling Systems and Methods”, which is acontinuation of U.S. application Ser. No. 16/368,291 filed on Mar. 28,2019, (now U.S. Pat. No. 10,855,127) entitled “Electric Machine StatorCooling Systems and Methods”, which is a divisional of U.S. applicationSer. No. 16/228,353 filed on Dec. 20, 2018, (now U.S. Pat. No.10,505,421) entitled “Electric Machine Stator Cooling Systems andMethods”, which is a divisional of U.S. application Ser. No. 15/870,472filed on Jan. 12, 2018, (now U.S. Pat. No. 10,177,631) entitled“Electric Machine Stator Cooling Systems and Methods”, which claimspriority to U.S. Patent Application Ser. No. 62/570,441, filed on Oct.10, 2017, entitled “Permanent Magnet Motor with Tested Efficiency BeyondUltra-Premium/IE5 Levels”. The U.S. Ser. No. 16/368,291 application isalso a continuation of U.S. application Ser. No. 15/870,458 filed onJan. 12, 2018, (now U.S. Pat. No. 10,256,688) entitled “Electric MachineRotor Cooling Systems and Methods”, which claims priority to U.S.Application No. 62/570,441 filed on Oct. 10, 2017, entitled “PermanentMagnet Motor with Tested Efficiency Beyond Ultra-Premium/IE5 Levels”.The U.S. Ser. No. 16/368,291 application is also a continuation of U.S.application Ser. No. 15/870,420 filed on Jan. 12, 2018, (now U.S. Pat.No. 10,256,700) entitled “Robust Permanent Magnet Electric Machine andMethods”, which claims priority to U.S. Application No. 62/570,441 filedon Oct. 10, 2017, entitled “Permanent Magnet Motor with TestedEfficiency Beyond Ultra-Premium/IE5 Levels”, the content of which parentapplications are incorporated herein in their entirety for all purposes.

COPYRIGHT STATEMENT

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD

The present disclosure relates, in general, to methods, systems, andapparatus for cooling the stator of an electric machine, for example anelectric motor or electric generator. The present disclosure relatesmore specifically to the cooling of the stator of a permanent magnet(PM) electric machine.

BACKGROUND

Electric machines employed in commercial and industrial applications areoften required to operate at 100% of the applicable power rating overwhat is typically a 60,000-hour service life. Therefore, a motor orother electric machine employed in a commercial or industrial settingmust be both reliable and versatile. Machine reliability and lifespancan be compromised by many factors including but not limited to: 1) hightemperature operation resulting in thermally induced component failures,2) vibration, heat, friction, unbalanced operation, contamination,inadequate material selection or other causes of mechanically inducedcomponent failure, or 3) dielectric failure resulting in electricalshort circuits or open circuits caused by heat, inadequate materialselection or other causes.

Electric machines are often provided with a mechanical cooling system,for example a fan plus a cowling configured to direct air over coolingfins cast into the machine housing. Many such devices are classified astotally enclosed fan cooled (TEFC) devices. TEFC motors suffer frominherent inefficiency since a portion of the total power output by themotor must turn the cooling fan, and therefore is unavailable foroutput. For example, a TEFC motor attached to a conveyor belt mustoperate both the cooling fan and the conveyor belt, therefore theportion of the motor output required to turn the fan is not available atthe conveyor belt. The fan and cowling apparatus of a TEFC machine isprone to damage, and presents a safety risk in a typical industrialsetting. Furthermore, since a TEFC machine drives a cooling fan with oneside of the motor shaft, it is impossible or difficult to attach twodownstream machines to a single TEFC motor.

Conventional electric machines are made robust and durable usually byimplementing the machines with large and heavy components. The largerand heavier components required to promote stable long-term operationcan increase machine cost and weight.

Other electric machines, having directly ventilated and thereforepartially open housings to promote cooling, can suffer from moisture andparticulate matter contamination which in turn can cause mechanicalcomponent degradation and wire insulation or electric junctiondegradation and premature failure. Totally enclosed non-ventilated(TENV) electric machines present unique cooling challenges since TENVdevices lack direct ventilation or an external fan moving air overexternal cooling fins.

The embodiments disclosed herein are designed to minimize one or more ofthe above problems.

SUMMARY

Embodiments disclosed herein include electric machine cooling apparatusand methods. Other embodiments include elements imparting enhancedrobustness and durability to an electric machine. As used herein, theclass of devices referred to as electric machines includes both electricgenerators and electric motors. Certain embodiments described herein arepermanent magnet motors having a radial flux configuration. Many of thedisclosed methods and apparatus are also applicable to transfer heat orin part robustness to axial flux machines, transfer flux machines, andlinear machines. Certain methods and apparatus may be applicable tonon-rotating torque motors, transformers, or inductors. Although manyspecific embodiments are illustrated with respect to permanent magnetmotors, the disclosure and claims are not limited to any specificapparatus configuration and are applicable to any type of electricmachine.

Certain embodiments disclosed herein include an electromagnet having astack of laminations defining a tooth and a yoke segment, and aninsulating bobbin surrounding a portion of the tooth of each lamination,such that each lamination is held against adjacent laminations by thebobbin. The disclosed electromagnets also have electrically conductivewindings surrounding a portion of the bobbin; and an encapsulant fullyencapsulating the bobbin and windings, wherein the encapsulant comprisesa dielectric material and an additive to increase the thermalconductivity of the encapsulant.

In some embodiments, the dielectric material comprises a polymer and theadditive comprises one or more of boron nitride, silicon carbide,silicon; aluminum powder, copper powder, metal oxide, ceramic, andgraphene.

In some embodiments, Individual laminations in the stack of laminationsmay include opposing first and second planar faces, with one face beingin thermal contact with a heat transfer layer. Individual laminations inthe stack of laminations may further be in physical contact with adielectric layer, opposite the heat transfer layer.

Other embodiments include a method of fabricating an electromagnet orstator having a plurality of electromagnets as described above. In someembodiments, each yoke segment defined by a stack of laminations furtherdefines a tongue structure and an opposing groove structure. A pluralityof electromagnets may be assembled into a stator by mating the tonguestructure and the groove structure of each electromagnet with thecorresponding tongue structure and the groove structure of adjacentelectromagnets; and encapsulating the assembled stator with a thermallyconductive encapsulant.

Certain electric machine embodiments disclosed herein minimize theproduction of heat caused by eddy currents in the permanent magnets,rotor back assembly, electromagnetic cores, or other structures. Inaddition, methods and apparatus are disclosed providing for the removalof heat from an electric machine by conduction, convection and radiationutilizing various heat paths through the internal cavities andcomponents or subsystems of the machine.

An embodiment disclosed herein is an electric machine having a rotor, astator and a housing. The rotor includes a shaft defining a lengthwiseaxis. The shaft is surrounded by a rotor back assembly, also known as aback-iron assembly. The rotor also includes a radially mounted array ofpermanent magnets positioned around the perimeter of the rotor backassembly. The machine stator includes a plurality of electromagnetsradially positioned around the rotor defining an air gap between anexterior surface of the permanent magnets of the rotor and an interiorsurface of the electromagnets of the stator.

The rotor and the stator are supported by and enclosed within a housing.In certain embodiments the housing is a totally enclosed nonventilated(TENV) housing. In one embodiment, the stator and the housing define asubstantially cylindrical rotor cavity within the air gap and bounded bythe housing end plates or similar structures. The rotor cavity mayfurther be divided into first and second cavities when the rotor ispositioned within the rotor cavity. Specifically, a first cavity existsbetween the rotor and the housing at one end of the rotor, adjacent to ahousing end plate. A second cavity exist between the rotor and housingat the other end of the rotor, adjacent the other end plate. The housingend plates may be separate plates attached to a housing perimeterportion, alternatively one of the end plates and the housing perimetermay be a cast, machined or otherwise unitary or co-formed housingelement.

The first and second cavities are connected through the air gap. Inaddition, one or more ventilation channels can be provided through therotor back assembly, extending from the first cavity to the secondcavity. The first and second cavities, the air gap, and the ventilationchannels therefore define a fluid circuit around the exterior of andthrough the back-iron of the rotor. The rotor may also include aninternal fan extending into the first or second cavity. The internal fanis part of the rotor or connected to the rotor and is configured tocause low pressure at either the air gap or the ventilation channels andhigh pressure at the other of the air gap and ventilation channels.Therefore, when the rotor rotates, air or another fluid, for example anair and oil mixture, is caused to circulate around the fluid circuitfrom one cavity through the air gap to the other cavity and back to theoriginal cavity through the ventilation channels. Thus, heat generatedin the rotor during operation can be transferred to the air or otherfluid flowing in the fluid circuit to cool the rotor.

The rotor fan can be a separate structure, or could be rotor fan bladesformed in an exterior surface of the rotor back assembly. In addition,the rotor back assembly, fan blades, or fan may be treated to enhancethe radiation of heat to the fluid circuit and therefore to the firstcavity or the second cavity. Suitable surface treatments include but arenot limited to surface roughening, or surface anodization.

Heat is produced during the operation of an electric machine within therotor primarily by magnetically induced eddy currents in the permanentmagnets and magnetically induced eddy currents and hysteresis within therotor back assembly. In certain embodiments, the production of heatwithin the rotor can be reduced by implementing both the permanentmagnets and the rotor back assembly with a series of laminations.Structural support and advantageous heat transfer characteristics may beprovided by binding the permanent magnets to the rotor back assemblywith a retainer band surrounding an outer surface of the magnets, facingthe air gap.

Heat transfer from the rotor to the fluid circuit and subsequent heattransfer from the first and second cavities to the machine housing maybe facilitated with various heat transfer structures. One class of heattransfer structures is mounted to the rotor facing either the firstcavity or the second cavity. Another class of heat transfer structuresmay be mounted to the housing, typically at the end plates, facing intothe first cavity and/or the second cavity. Any one of these heattransfer structures may include an array of pins, fins, combinationpin/fins, or other structures to increase surface area and turbulence,and therefore promote effective heat transfer to or from the attachedrotor or housing structure. In addition, a heat transfer structure maybe fabricated from a material such as aluminum or copper havingrelatively high thermal conductivity. A heat transfer structure may betextured, colored, have a surface treatment, or otherwise fabricated toeffectively transfer heat to or from the fluid circuit.

Additional heat may be transferred away from the permanent magnets byincluding a thermally conductive filler or encapsulant material in thegap between adjacent permanent magnets. The thermally conductiveencapsulant material may be a polymer such as an epoxy having anadditive suspended within the polymer matrix to increase the thermalconductivity of the rotor encapsulant above the native thermalconductivity of the polymer, epoxy or other rotor encapsulant material.The thermally conductive encapsulant serves to conduct heat away fromthe sides of the permanent magnets toward the first and second cavitiesduring rotor operation. Any heat transfer structures attached to therotor can be placed into thermal contact with the thermally conductiveencapsulant regions to promote heat exchange with the fluid circuit.

A thermally conductive encapsulant in the gap between adjacent permanentmagnets also provides structural rigidity and robustness to the rotor.The encapsulant serves to additionally secure the permanent magnets tothe rotor back assembly and prevent the magnets from slipping around thecircumference of the rotor under load. In some embodiments, the rotorback assembly may define an anchoring surface between adjacent permanentmagnets serving to more securely anchor the encapsulant to rotor backassembly. An anchoring surface may be a groove, protrusion, keyway orthe like formed in her extending from the rotor back assembly.

A portion of the heat generated in the permanent magnets or rotor backassembly may be conducted to the shaft and conducted from the shaft toequipment driving or being driven by the electric machine. A machineshaft is typically fabricated from steel or another high-strength alloythat may not have relatively high thermal conductivity. The thermalconductivity of a machine shaft may be enhanced by providing the shaftwith a thermally conductive shaft core made of a material, copper forexample, having a different composition, and higher thermal conductivitythan other portions of the shaft.

In certain embodiments heat may be transferred to the shaft from thehousing as well. The shaft is typically supported by bearings at eachend of the rotor. The bearings are supported by the housing. In someembodiments portions of the bearing structure, the bearing seals forexample, may be fabricated from a material having enhanced thermalconductivity such as copper. Bearing flanges or other housing elementssupporting the bearings may also be fabricated from a material havingenhanced thermal conductivity. In such embodiments, the thermallyconductive shaft core may be made to extend toward the shaft perimeterwhere the shaft and bearings are in contact.

It may be advantageous in certain embodiments to seal the housing. Forexample, a TENV motor may be sealed to prevent internal contamination.In such an embodiment the bearings may be accessible from outside thehousing to facilitate bearing removal or replacement without requiringthe housing to be opened.

Electric machine embodiments also include a stator having a plurality ofradially positioned electromagnets. In some embodiments, the stator isencapsulated such that the stator encapsulant is in thermal contact withhousing structures, for example the housing end plates. In someembodiments the entire perimeter portion of the housing is in thermalcontact with the stator or the stator encapsulant. In an embodiment, thehousing includes a perimeter portion, a first end plate at the secondend plate. The end plates may be separate structures or co-fabricatedwith the perimeter portion of the housing. The stator encapsulant maycontact the first end plate and the second end plate such that a centralregion of the first end plate, a central region of the second end plate,and an interior stator surface define the enclosed cylindrical rotorcavity. In some embodiments, substantially no voids will extend from theinterior stator surface, the central region of the first end plate, andthe central region of the second end plate toward the perimeter portion.

The stator encapsulant provides for device robustness and thermaltransfer from the stator to the housing. The thermal conductivity of theencapsulant may be enhanced by mixing an additive to the encapsulant toincrease the thermal conductivity of the encapsulant. For example, theencapsulant may be a dielectric material such as a polymer or epoxy andthe additive may be boron nitride, silicon carbide, silicon, aluminumoxide, aluminum powder, copper powder, metal oxide, ceramic, graphene,substantially spherical particles or combinations of these or similarmaterials.

Thermal transfer between the stator and the housing and overall machinerobustness may be enhanced by fitting the stator closely to theperimeter portion of the housing. Thermal transfer between the statorand the perimeter portion of the housing may be further enhanced byfilling any gap between the stator perimeter and housing with athermally conductive lubricant or encapsulant.

Thermal transfer from the rotor cavity defined by the stator and housingmay be facilitated by providing one or more heat transfer structures inthermal contact with the central regions of the housing or an end plate.The heat transfer structures may include pins, fins, combinationpin/fins or other structures extending into the rotor cavity to increasesurface area, air turbulence, or otherwise promote heat transfer fromthe rotor cavity. In addition, any heat transfer structure may becolored, anodized, or have a surface treatment designed to promoteeffective heat transfer.

Thermal energy transferred to the housing may be removed from theelectric machine by conduction, convection or radiation. Heat transferfrom the housing may be enhanced by providing the housing with fins,external heat transfer structures, black anodization or other means. Inaddition, the housing may include feet fabricated from a material havinghigh thermal conductivity, aluminum for example. Heat may be transferredfrom the feet to a mounting surface, for example a factory floor, shelf,or other equipment. Heat transfer from the feet to the mounting surfacemay be facilitated by providing an interface having high thermalconductivity between the feet and the mounting surface, for examplethermal paste or copper.

Stator embodiments include a radial array of electromagnets. In certainembodiments, each electromagnet includes a core having a stack oflaminations defining a tooth portion and a yoke segment. An insulatingbobbin may be provided surrounding a portion of the tooth of eachlamination. Electrically conductive windings then surround a portion ofthe bobbin. In one embodiment, each lamination in the stack oflaminations is held against adjacent laminations solely by pressure fromthe bobbin, without the use of screws, welds, pins, crimp joints, glueor other fastening means.

In some embodiments, heat transfer from an electromagnetic core may beenhanced by providing a heat transfer layer in thermal contact with oneor both of the planar faces defined by a core lamination. The heattransfer layer may be any material having a higher thermal conductivitythan the magnet steel used to fabricate the laminations. Representativeheat transfer materials include, but are not limited to, metals such ascopper, nickel, silver or materials such as graphene. The heat transfermaterial must be in thermal contact with the associated lamination,meaning that heat from the lamination may transfer directly to the heattransfer material. Thermal contact may be physical contact.Alternatively, thermal contact may occur through an intermediatematerial such as a thermal paste. In some instances, the heat transfermaterial may be deposited on, plated onto, coated onto or otherwisepermanently bonded to the lamination.

Electromagnetic cores will also typically include a dielectric layerbetween laminations. In one embodiment, laminations will have adielectric layer applied or in contact with one planar face and a heattransfer layer in thermal contact with the opposing planar face. In thisembodiment the interface between adjacent laminations will include adielectric layer from one lamination and a heat transfer layer from theother lamination.

The arc-shaped yoke segment defined by a stack of laminations may incertain embodiments define a tongue structure and an opposing groovestructure configured to mate with each other. Thus, a stator may beassembled from a plurality of electromagnets by engaging the tonguestructure of the first electromagnet with the groove structure of anadjacent electromagnet and so on until the stator is completed. Incertain embodiments, the yoke segments of a plurality of electromagnetsis directly supported by a shoulder structure extending from the housingor a housing end plate providing machine robustness and a direct thermalpathway from the stator to the housing.

Alternative embodiments include methods of cooling an electric machinerotor, methods of cooling an electric machine stator, methods of coolingan electric machine, methods of fabricating an electric machine, methodsof stabilizing an electric machine, and methods of fabricating anelectromagnet for an electric machine.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of particularembodiments may be realized by reference to the remaining portions ofthe specification and the drawings, in which like reference numerals areused to refer to similar components. In some instances, a sub-label isassociated with a reference numeral to denote one of multiple similarcomponents. When reference is made to a reference numeral withoutspecification to an existing sub-label, it is intended to refer to allsuch multiple similar components.

FIG. 1 is an isometric view of a representative electric machine; atotally enclosed nonventilated (TENV) permanent magnet (PM) motor.

FIG. 2 is an isometric cross-sectional view of the motor of FIG. 1.

FIG. 3 is an isometric shaft end (SE) view of a rotor showing permanentmagnets and other structures.

FIG. 4 is an isometric opposite shaft end (OSE) view of the rotor ofFIG. 3.

FIG. 5. is an isometric opposite shaft end (OSE) view of an alternativerotor embodiment.

FIG. 6 is an isometric SE view of the rotor of FIG. 3 showing a retainerband around the rotor structure.

FIG. 7 is an isometric OSE view of the rotor of FIG. 6.

FIG. 8 is a side elevation cross-sectional view of the motor of FIG. 1.

FIG. 9 is an enlarged view of a portion of the motor of FIG. 8 showing aforced fluid circuit.

FIG. 10 is an isometric view of portions of the housing and stator ofthe motor of FIG. 1.

FIG. 11 is an isometric view of portions of the housing and stator ofthe motor of FIG. 1 showing a stator encapsulant.

FIG. 12 is an isometric cross-sectional view of the motor of FIG. 1 withthe rotor removed.

FIG. 13 is an isometric view of portions of the housing and stator ofthe motor of FIG. 1 with selective electromagnet portions removed.

FIG. 14A is an isometric view of an electromagnet structure.

FIG. 14B is an exploded isometric view of the electromagnet structure ofFIG. 14A.

FIG. 15A is a schematic diagram showing a layered EM laminationstructure.

FIG. 15B is a schematic diagram showing an alternative layered EMlamination structure.

FIG. 15C is a schematic diagram showing an alternative layered EMlamination structure.

FIG. 16 is a side elevation cross-sectional view of the motor of FIG. 1showing detail at the interface between the housing and stator.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS Overview

Certain embodiments disclosed herein are electric machines engineered toprovide relatively maintenance-free, robust, long-term, and efficientservice in commercial, transportation, or industrial settings. As usedherein, the class of devices referred to as “electric machines” includesboth electric generators and electric motors. Certain embodimentsdescribed herein are permanent magnet motors having a radial fluxconfiguration. Many of the disclosed methods and apparatus are alsoapplicable to improve the overall robustness and thermal performance ofaxial flux machines, transfer flux machines, and linear machines.Certain methods may be applicable to the thermal management or stabilityof non-rotating torque motors, transformers, or inductors. Although manyspecific embodiments are illustrated herein with respect to totallyenclosed nonventilated (TENV) permanent magnet motors, the disclosureand claims are not limited to any specific apparatus configuration andare applicable to any type of electric machine.

Electric machines generate heat during operation. Heat, if notdissipated properly, can reduce the life of the machine significantly.Sustained operation at high temperatures can impact the physicalproperties of many machine components, including but not limited toelectrical insulation, electrical contacts, encapsulation materials,magnets and so forth. Excess heat can make these components soft whenfirst hot and then gradually brittle, impacting device performance andleading to premature failure. Accordingly, two keys to designing arobust electric machine are reducing the generation of heat duringmachine operation, and increasing heat dissipation from the machine.Many of the methods and apparatus described herein provide for one orboth of reduced heat production and effective heat dissipation from amachine during operation. The disclosed techniques and structures arecollectively referred to as thermal management methods or thermalmanagement apparatus.

Some methods and apparatus providing for advantageous thermal managementalso generally enhance the mechanical stability of a machine, andtherefore provide additional machine robustness and durability. Othermethods and apparatus described herein enhance machine stability androbustness without affecting thermal properties.

Certain TENV machines disclosed herein feature an overall deviceconfiguration designed to enhance thermal management and machinerobustness. For example, a representative TENV machine, the permanentmagnet motor 10 of FIG. 1, may have a pancake shape, with concentratedelectromagnet windings, maximized slot fill, surface mounted andoversized permanent magnets and other apparatus to enhance thermalmanagement and provide overall machine robustness as detailed herein.Specific embodiments of the disclosed permanent-magnet TENV motor 10 useless than half the copper and less than half the electric steel comparedto an induction motor of the same power rating.

In addition, several disclosed embodiments utilize aggressive, butpassive cooling. Multiple heat paths are designed into the machines toprovide for heat transfer from interior structures to the externalhousing where heat may be dissipated by natural convection into thesurrounding air, by radiation to surrounding objects, by conduction intomachine mounting surfaces, and/or by conduction through the drive shaftinto a driven device (for example a fan, pump, conveyor belt, wheels, orother apparatus).

FIG. 1 is an isometric external view of a representative electricmachine, permanent-magnet TENV motor 10. FIG. 2 is an isometric crosssection view of the motor 10 showing certain internal elements. Themotor 10 includes a housing 12, surrounding and supporting internalcomponents and a shaft 14 connected to a rotor 16. The housing 12 may beconnected to or include various supporting structures, which can besupplied or varied according to need. For example, the housing may beconnected to feet 18, lifting eyebolts 20, a C-face, a flange-face, orother supporting or attachment structures facilitating the placement andmounting of the motor 10 into an operational setting.

As shown in FIG. 2, the shaft 14 is connected to the rotor 16 such thatthe shaft 14 defines a lengthwise shaft axis 22 around which the shaft14 and rotor 16 rotate when the motor 10 is operated. Alternatively,external torque applied to the shaft 14 can cause the rotor to rotatearound the shaft axis 22 if the electric machine is a generator. Theshaft 14 and rotor 16 are supported by bearings 24 and 26 seated inbearing flanges 28 and 30. The embodiment illustrated in FIGS. 1 and 2features a shaft 14 extending through the housing 12 from only one sideof the rotor 16. Alternative embodiments may include a shaft 14extending through the housing 12 from both sides of the rotor 16. Suchan alternative embodiment, which is not feasible with a TEFC machinebecause of the cooling fan, can advantageously drive two downstreammachines at once, with one downstream machine being attached to each endof the shaft 14. In embodiments featuring a shaft 14 extending from oneside of the rotor 16 only, the opposing ends of the motor 10 may bereferred to as the shaft end “SE” and opposite shaft and “OSE” forconvenience. Thus, elements such as bearings 24 and flange 28 may bereferred to herein as the SE bearings 24 and the SE bearing flange 28respectively. It is important to note however, that this disclosureexpressly covers electric machines having shafts 14 extending from oneside of the housing 12, both sides of the housing 12, or not extendingfrom the housing 12 at all.

The rotor 16 is substantially surrounded by a stator 32. As described indetail herein, the rotor 16 includes a series of permanent magnets 34arranged around, but spaced away from the shaft axis 22. The permanentmagnets 34 are supported by a rotor back assembly 36 sometimes referredto as a back-iron assembly because this assembly is typicallyconstructed of a magnetic material such as steel or another type ofsteel/iron alloy. The rotor back assembly 36 is mechanically bonded tothe shaft 14 or co-fabricated with the shaft.

The stator 32 includes a series of electromagnets 38 surrounding therotor 16 such that the electromagnets 38 and permanent magnets 34 areseparated from each other by an air gap 40. In highly simplified terms,motor operation occurs when alternating current is applied to thewindings 42 of the electromagnets 38, causing a varying magnetic fieldto be formed by the stator 32. Magnetic attraction between the permanentmagnets 34 and the electromagnets 38, within the varying magnetic field,causes the rotor 16 to rotate with respect to the stator 32. Thus,torque may be transferred to any device(s) attached to the shaft 14 asis typical with motors. In an alternative generator configuration, theshaft 14 may be rotated by an external source of torque, causing thepermanent magnets 34 to form a varying magnetic field. The varyingmagnetic field can then induce alternating current in the windings 42,thereby generating electricity.

Heat Generation in Electric Machines

When the motor 10 or other electric machine is operated, heat is createdin both the rotor 16 and stator 32. The principal sources of heatgenerated in the rotor 16 are eddy current losses in the permanentmagnets 34 and eddy current losses or hysteresis losses in the rotorback assembly 36. The principal source of heat generated in the stator32 include resistance in the windings 42 and eddy current/hysteresislosses in the associate electromagnet cores 44. Furthermore, drag, alsodescribed as windage, is created as the rotor 16 rotates within themotor 10. Windage generates additional heat. Friction at the surfaces ofbearings 24 and 26 also creates heat inside the housing 12. As notedabove, a certain class of electric machine is described as a totallyenclosed and non-ventilated “TENV” machine or motor. A TENV motorprovides certain advantages, including but not limited to reducedmaintenance requirements, since the internal motor elements aresubstantially sealed against external contamination. Heat generatedwithin a sealed TENV machine must be dissipated however, without anexternal fan circulating air over the housing and without directventilation openings to avoid premature component failure.

The disclosed apparatus and methods of facilitating thermal managementin an electric machine, and therefore promoting general machinerobustness, can be classified as either (a) methods and structures forminimizing the production of heat, or (b) methods and structuresfacilitating machine cooling after heat has been produced. Severalthermal management techniques described herein involve the export ofheat through the rotor 16, stator 32 and/or housing 12. Severalalternative thermal management strategies are described herein. Thevarious methods and apparatus may be combined with one another in anyfashion, scaled, or partially implemented as necessary to achievespecific heat mitigation goals.

Electric Machine Rotor Structure

As noted above, the production of heat during the operation of anelectric machine is inevitable but can, in certain instances be reduced.The primary sources of heat generation in the rotor 16 are magneticallyinduced eddy currents within the permanent magnets 34 and magneticallyinduced eddy currents or hysteresis losses within the rotor backassembly 36. The scale of each type of magnetic eddy current and theresulting heat production may be reduced by implementing the permanentmagnets 34 and rotor back assembly 36 as laminated structures.

For example, FIGS. 3-5 are isometric views of two alternativeembodiments of a rotor 16. The first embodiment of rotor 16, shown inFIGS. 3 and 4, is the rotor 16 from FIG. 2. This rotor 16 features ashaft 14 extending from only one side. Alternative embodiments include ashaft 14 extending from both sides of the rotor 16. The FIG. 3-4 rotorembodiment is shown in a SE isometric view in FIG. 3 and an OSEisometric view in FIG. 4. The alternative rotor 16 of FIG. 5 could beimplemented with a single or dual shaft configuration. Each rotor 16features a rotor back assembly 36 mechanically bonded around a portionof the shaft 14. Permanent magnets 34 are mounted around a perimeter of,and in contact with the rotor back 36 such that the permanent magnets 34are radially arranged around, but spaced away from, the shaft 14.

A permanent magnet 34 may be fabricated from any number of laminations44. Laminations 44 are fabricated from the permanent magnet material,which may be a rare-earth magnet material, for example aneodymium-iron-boron magnet material, a samarium-cobalt magnet material,Alnico magnets, and the like, or a conventional magnet material such asa ferrite ceramic. In one representative embodiment, the permanentmagnets 34 of rotor 16, as illustrated in FIG. 5, have twenty-four (24)laminations 46. Each lamination 46 is a relatively thin, planar sectionof permanent magnet material with multiple laminations being stacked oneon top of the other such that the plane defined by the interface betweenadjacent laminations is generally perpendicular to the shaft axis 22.Alternative embodiments of permanent magnet 34 may include any number oflaminations 46, for example a permanent magnet 34 may include 2, 4, 8,12, 16, 20, 24, 28, 32, 36, 40 or more laminations 46 to reduce thescale of magnetically induced eddy currents and heat production. Eachlamination 46 within a permanent magnet 34 may optionally be separatedfrom adjacent laminations 46 by an insulator, such as a lacquer,varnish, paper, or other relatively thin insulating material.

Relatively high-performance rare-earth magnet materials may be selectedfor the permanent magnets 34 of the rotor 16. Rare-earth magnets havehigher remanence, much higher coercivity and energy product than otherpermanent magnet types. Thus, machine efficiency can be enhanced withrare-earth permanent magnets 34, although steps must be taken to promoteoverall machine robustness and stability if rare-earth permanent magnets34 are utilized.

Specifically, rare-earth magnets can be demagnetized if they become toohot, and the magnetic properties of rare earth magnets will not recoverwhen the magnets cool down. Therefore, rare-earth magnets 34 must beselected with a higher temperature rating than the maximum temperatureanticipated in the permanent magnets 34 during thermally stableoperation at the highest rated power output. For example, if theexpected high temperature of the permanent magnets 34, according to aselected design, is 130° C., then it is advisable to utilize rare-earthmagnets 34 that are temperature rated to at least a 35% higher,temperature (according to UH grade), for example a up to 180° C., toprovide operational headroom.

Rare-earth permanent magnets can also be demagnetized by excessive fluxgenerated by large currents in the stator windings 42. Therefore,selected embodiments of motor 10 utilize permanent magnets 34 withgeometries that create a large permeance coefficient to increaseresistance to flux-based demagnetization. For example, as shown in FIGS.3 and 4, motor 10 may include a rotor 12 having rare-earth permanentmagnets 34 with a radial thickness dimension w, measured along a radiusline extending outward from the shaft axis that is eight (8) times orgreater than the width of the magnetic air gap 40 measured along thesame radius line. The use of rare-earth permanent magnets 34 having ahigh permeance coefficient allows the motor to operate in conditions farbeyond nominal ratings without threat of demagnetization. Some of theseconditions could include operation at peak torque, operation at extendedspeed ranges utilizing field weakening, or a combination of both.

The rotor back assembly 36 may also be assembled from multiplelaminations of steel, iron, another iron alloy, or another suitablerotor back assembly material. In one representative embodiment, shown inFIG. 5, the rotor back assembly 36 includes six (6) laminations 50. Eachlamination 50 is a relatively thin, flat, annular section of rotor backmaterial. Multiple laminations are stacked one on top of the other withthe plane defined by the interface between adjacent laminations 50 beinggenerally perpendicular to the shaft axis 22. Alternative embodiments ofrotor back assembly 36 may include any number of laminations 50, forexample, the rotor back 36 may include 2, 4, 8, 12, 16, 20, 24, 28, 32,36, 40 or more laminations 50 to minimize the scale of magnetic eddycurrents, hysteresis loss, and heat production within the rotor backassembly 36. Each lamination 50 of the rotor back assembly 36 mayoptionally be separated from adjacent laminations 50 by an insulator,such as a lacquer, varnish, paper or other relatively thin insulatingmaterial.

During operation, the rotor 16 rotates at a high rate of speed and issubject to varying magnetic flux. Therefore, it is important to assurethat the permanent magnets 34 are securely bonded to the rotor backassembly 36. An adhesive may optionally be used to bond the permanentmagnets to the rotor back assembly 36. In certain embodiments, asillustrated in FIGS. 6 and 7, the rotor 16 includes a retainer band 52around the perimeter of the rotor 16 facing the air gap 40 and stator32. The retainer band 52 can be prestressed to secure the permanentmagnets 34 and adjacent structures during operation. In addition, theretainer band 52 may be specifically configured to minimize drag as therotor 16 rotates, and thereby minimize windage heat production.

In certain embodiments, the retainer band 52 is fabricated from amagnetic material such as steel or a graphene composite. In suchembodiments, the banding may be implemented from a plurality ofseparated bands to minimize the generation of eddy currents in the band52. In addition, the banding may be impregnated with a heat transfermaterial or otherwise treated to facilitate heat transfer from thepermanent magnets 34 to the outside surfaces of the band 52 and air gap40. Alternatively, the retainer band 52 may be fabricated entirely froma material selected to have enhanced heat transfer properties, forexample copper or aluminum. Alternatively, the band 52 may be fabricatedentirely from a carbon fiber mat or carbon fiber filament that can beprestressed, does not generate eddy currents, and also has relativelyhigh thermal conduction properties.

Rotor Cooling Methods and Apparatus

Heat production in an operating electric machine rotor can be reducedusing the techniques described above, but some heat production isinevitable. Therefore, several apparatus and methods are disclosedherein for cooling an electric machine rotor. It is important to notethat the rotor 16, particularly in a TENV machine such as the motor 10,is substantially or entirely enclosed within the machine housing 12 andsurrounded by the stator 32. Therefore, cooling a rotor 12 ofteninvolves heat transfer to another motor structure prior to heat exportfrom the motor 10. In certain instances, the rotor cooling methods andapparatus described herein operate in conjunction with methods andapparatus for cooling other portions of the motor 10, stator 32 and/orhousing 12.

A. Rotor Forced Fluid Circuit

FIG. 8 is a side elevation cross-sectional view of the motor 10 shown inFIG. 2. FIG. 9 is an enlarged view of a portion of the rotor 16, housing12, and stator 32 shown in FIG. 8. During operation, the rotor 16 mustbe permitted to spin freely within the stator 32 and housing 12. Thus,the rotor 16, stator 32, and housing 12 collectively define certaincavities within which the rotor 16 operates. For example, the housing 12of FIGS. 1-2 and 8-9 includes a perimeter portion 54 surrounding thestator 32, and therefore surrounding the shaft axis 22. The perimeterportion 54 of the housing 12 is substantially closed, except for one ormore sealed shaft openings, by a first end plate 56 and an opposing endplate 58 at each end of the perimeter portion 54. Thus, the perimeterportion 54, first end plate 56 and opposing end plate 58 define theoverall pancake shape of the motor 10. The perimeter portion 54, firstend plate 56, and second and plate 58 may be separate structures thatare bonded together to form a housing 12. Alternatively, the perimeterportion 54 and one end plate 56, 58 or other housing structures may becast, machined or otherwise formed as a single part, with the perimeterportion 54 and end plate 56 or 58 serving to identify different regionsof a single housing structure.

Is best viewed in FIGS. 10 and 11, portions of the stator 32 facing theair gap 40 and the end plates 56, 58 define a substantially cylindricalrotor cavity 60. Certain embodiments of the motor 10 include apparatusdesigned to force air circulation between various distinct regions ofthe overall cylindrical rotor cavity 60 as described below.

Specifically, an open space between one end 62 of the rotor 16 andadjacent portions of the housing 12 defines a substantially annularfirst cavity 64 within the cylindrical rotor cavity 60. Similarly, theopen space between the opposite end 66 of the rotor 16 and adjacentportions of the housing 12 defines a substantially annular second cavity68 within the cylindrical rotor cavity 60. Furthermore, the relativelythin air gap 40 extends between the outer perimeter of the rotor 16 andthe inwardly facing surfaces of the stator 32 to complete thecylindrical rotor cavity 60. As best shown in FIGS. 2 and 4, air, anair/oil mixture, another gas, liquid, or a mixed fluid may be caused tocirculate from one of the cavities 64, 68 to the other cavity 64, 68 andthrough the air gap 40 by providing the rotor 16 with an internal fansurface such as internal fan 70, and one or more ventilation channels 72through the rotor 16.

Specifically, a series of ventilation channels 72 can be providedthrough the rotor back assembly 36 as best illustrated in FIG. 3 andFIG. 4. In the illustrated embodiment, each ventilation channel 72defines a portion of an arc around the shaft axis 22, and constitutes anopening extending through each lamination 50 of the rotor back assembly36. Other shapes and configurations of ventilation channel 72 are withinthe scope of this disclosure, provided each ventilation channel 72 hasan opening in fluid communication with the first cavity 64 and thesecond cavity 68.

The first cavity 64, second cavity 68, air gap 40, and each ventilationchannel 72 together define an internal forced fluid circuit 74 in partsurrounding and extending through the rotor 16. Air, another fluid, oras described below, an air and oil mixture may be caused to circulatethrough the internal forced fluid circuit 74 by the internal fan 70. Thefan 70 may be part of, attached to, or driven by the rotor 16 to causeair or another fluid to circulate within the internal forced fluidcircuit 74. Specifically, the fan 70 includes a plurality of fan blades76 configured to cause a relatively low-pressure zone at the air gap 40and a relatively high-pressure zone toward the shaft 14 at the secondcavity 68, when the rotor 16 rotates in a clockwise direction, as viewedin FIG. 4.

This pressure differential causes air or another fluid to circulate fromthe second cavity 68 through the ventilation channels 72 to the firstcavity 64. Simultaneously, air or another fluid, is caused to circulatefrom the first cavity 64 through the air gap 40 to the second cavity 68,completing the forced fluid circuit 74. A different fan configuration ordifferent rotation direction could cause the air or other fluid tocirculate in the opposite direction.

In the embodiment of FIG. 4, the fan 70 extends into the second cavity68. In alternative embodiments, the fan 70 may extend into the firstcavity 64 or separate fans may extend into both cavities 64 and 68. Incertain embodiments, the fan 70 and/or fan blades 76 are a separatestructure attached to or driven by the rotor 16. In alternativeembodiments, the fan 70 may comprise a plurality of fan blades 76 formedinto the rotor back assembly 36, formed in a portion of the shaft 14, orotherwise attached to the rotor 16. In any embodiment, the fan 70 causesair or another fluid to circulate around and through the rotor 16completing the forced fluid circuit 74 as the rotor rotates.

Air or another fluid circulating within the forced fluid circuit 74 isheated by heat generated within the rotor 16 as described above, thuscooling the rotor 16. The heated fluid can transfer said heat to anotherstructure to ultimately cool the motor 10. Various structuresfacilitating heat transfer from the rotor 16 to the forced fluid circuit74 and beyond are described below. In addition, various structuresassociated with the rotor may have surface treatments designed topromote efficient heat transfer from the rotor 16 to the forced fluidcircuit 74. For example, any rotor structure, including but not limitedto the fan 70, fan blades 76, shaft 14, rotor back assembly 36, retainerband 52, or other structures may be roughened to increase surface areaor treated, for example with black anodization, to facilitate heattransfer between the rotor and the forced fluid circuit 74.

Heat transfer from the rotor 16 to the forced fluid circuit 74, or fromthe forced fluid circuit 74 to other motor structures such as thehousing 12, and ultimately away from the motor 10, may be facilitatedwith supplemental heat transfer structures. For example, as shown inFIG. 5, the rotor back assembly 36 or another rotor structure may beplaced into thermal contact with one or more heat transfer structures,for example SE heat transfer structure 78 and OSE heat transferstructure 80 shown in FIGS. 5, 8, and 9. As defined herein, “thermalcontact” means contact between two or more structures such that thermalenergy may flow from one structure to another structure. Structures indirect thermal contact with each other are also in physical contact witheach other. Alternatively, thermal contact may occur through anintermediate material such as a thermal paste. The SE heat transferstructure 78 and OSE heat transfer structure 80 are merelyrepresentative examples of any number of types or configurations of heattransfer structure that can be mounted to, formed in, or otherwisethermally contacted with the rotor 12. In each case, a rotor heattransfer structure 78 or 80 contacts the rotor on one side and extendsinto either the first cavity 64 or the second cavity 68 to facilitateheat transfer between the rotor 16 and the forced fluid circuit 74.

Other heat transfer structures may be bonded to or formed in thermalcontact with the housing 12 to facilitate heat transfer from the forcedfluid circuit 74 to the housing 12 and subsequently out of the motor 10through heat radiation, conduction or convection. For example, as shownin FIGS. 2, 8, 9, and 10, one, two, or more heat transfer structures maybe mounted to the housing 12 extending into the first cavity 64 orsecond cavity 68 toward the rotor 12. In the representative, butnonlimiting example shown in the figures, the motor 10 includes an SEhousing heat transfer structure 82 and an OSE housing heat transferstructure 84 extending into the first cavity 64 and second cavity 68respectively. Each of the heat transfer structures 82, 84 is illustratedas being substantially annular, however other shapes and configurationsare within the scope of this disclosure.

The SE housing heat transfer structure 82 and OSE housing heat transferstructure 84 are merely representative examples of any number of heattransfer structure types or configurations that can be mounted to,formed in or otherwise thermally contacted with the housing 12. In eachcase, a housing heat transfer structure 82 or 84 contacts the housing 12on one side and extends into either the first cavity 64 or the secondcavity 68 to facilitate heat transfer from the forced fluid circuit 74to the housing 12.

In several of the embodiments illustrated in the figures, a heattransfer structure 78, 80, 82, or 84 may be formed having a number ofpins, fins, combination pin/fins 86 or other structures designed toincrease surface area and turbulence. The pin/fins 86 extend away fromthe housing 12 or stator 32 and into the adjacent cavity 64 or 68. Aheat transfer structure 78, 80, 82, or 84 may be roughened to increasesurface area or treated, for example with black anodization tofacilitate heat transfer to or from the heat transfer structure and theforced fluid circuit 74. In addition, one or more heat transferstructures 78, 80, 82, or 84 may be fabricated from a material such ascopper or aluminum with high thermal conductivity. The heat transferstructures 78, 80, 82, or 84 may be bonded to the adjacent rotor orhousing structure using heat transfer paste or another interfacefacilitating effective heat transfer from the heat transfer structure78, 80, 82, or 84 to or from the forced fluid circuit 74.

B. Rotor Encapsulation and Stabilization

In certain embodiments, for example as illustrated in FIG. 5, adjacentpermanent magnets 34 are separated by a gap 88. Additional heat transferfrom the rotor 16 and particularly from the sides of each permanentmagnet 34 may be provided by filling all or a portion of the gap 88 witha thermally conductive rotor encapsulant 90. As detailed below, thethermally conductive encapsulant 90 also reduces windage and providesmechanical stability to the rotor 16. Representative examples ofthermally conductive rotor encapsulant 90 include, but are not limitedto, epoxy, an engineered polymer, polyester, polyurethane, silicone, oranother plastic, flowable or formable material suitable for filling thegaps 88. Thermal management may be enhanced by providing an additive tothe thermally conductive rotor encapsulant 90 to enhance the thermalconductivity of the encapsulant material above the native thermalconductivity of the encapsulant without modification. Representativeadditives to enhance thermal conductivity include, but are not limitedto suspended particles of boron nitride, silicon carbide, silica,aluminum oxide, aluminum, copper, another metal, another metal oxide,ceramic, graphene and the like. The thermal conductivity of theencapsulant 90 may further be enhanced if the suspended particles arespherical, have radially oriented fibers or have another shape ororientation designed to facilitate thermal conductivity.

Alternatively, a specific portion or region of the thermally conductiverotor encapsulant 90 can be fabricated from a substance havingrelatively high thermal transmissivity. For example, the gap 88 could befilled with epoxy or another polymer bound within a metal shell 92, forexample a copper or aluminum shell, where the shell has higher thermalconductivity than the polymer. In another embodiment, the gap 88 couldbe filled with epoxy or another polymer surrounding a more thermallyconductive core, for example an aluminum or copper core, in contactthrough the encapsulant with a permanent magnet 34, rotor back assembly36, and/or the forced fluid circuit 74. In any embodiment, the thermallyconductive rotor encapsulant should possess a glass transitiontemperature and maximum operating temperature that is significantlyhigher than expected motor operating temperature.

The thermally conductive rotor encapsulant 90 also serves to providemechanical strength to the rotor 12 and therefore enhance the overallrobustness of the motor 10. For example, thermally conductive rotorencapsulant 90 may be contacted with and/or bonded to the rotor backassembly 36 between adjacent permanent magnets 34, to mechanicallyanchor the permanent magnets 34 and prevent them from slippingcircumferentially around the rotor back assembly 36 under heavy load.The bond between the thermally conductive rotor encapsulant 90 and rotorback assembly 36 may be enhanced mechanically by providing the rotorback assembly 36 with slots 94, grooves, keyways, roughened surfaces,holes, projections, or other structures at the interface between therotor back assembly 36 and the surface of the thermally conductive rotorencapsulant 90.

As best shown in FIGS. 3 and 4, the thermally conductive rotorencapsulant 90 may be contacted at either or both ends with one or morerotor-side supplemental heat transfer structures. The specificembodiment of FIGS. 3 and 4 includes an array of SE heat transferelements 78 in thermal contact with the thermally conductive rotorencapsulant 90 and extending into the first cavity 64. In addition, theFIG. 3-4 embodiment includes an array of OSE heat transfer elements 80in thermal contact with the other end of the thermally conductive rotorencapsulant 90 and extending into the second cavity 68.

The heat transfer structures 78 and 80 are merely representativeexamples of any number of heat transfer structures that can be mountedto, formed in or otherwise thermally contacted with one end or the otherof a thermally conductive rotor encapsulant 90. In each case, heattransfer structure 78, 80 extend into either the first cavity 64 or thesecond cavity 68 to facilitate heat transfer between the rotor 16 andthe forced fluid circuit 74.

In an alternative embodiment, one or more of the heat transferstructures 78, 80, 82, or 84 may also be formed to function as the fan70. For example, the combination pin/fins 86 of the heat transferstructure 78 of FIG. 5 may be angled or otherwise formed to cause apressure gradient causing fluid circulation through the forced fluidcircuit 74.

C. Heat Transfer Oil

In certain embodiments, the efficiency of heat transfer within the motor10 may be enhanced by utilizing a heat transfer fluid, in combinationwith or other than air. For example, a quantity of transformer oil oranother heat transfer fluid may be added to the first cavity 64 and/orsecond cavity 68. When the motor is not operated, the oil will pool inthe bottom of each cavity 64, 68 and fill, for example, the bottomquadrant of the air gap 40. As the rotor 16 spins, the permanent magnets34 are sequentially submerged in the oil bath and heat can be drawn fromall exposed faces of the permanent magnets 34 and retainer band 52.

As described in detail below, a stator encapsulant may be added betweenthe stator 32 and housing 12, in part to ensure that oil added to aninternal cavity 64, 68 must pool in in contact with the rotor 16. See,for example, the stator encapsulant 96 of FIG. 11. Sufficient oil may beadded to the cavities 64, 68 to cover the air gap 40 and/or submergeportions of one or more of heat transfer structures 78, 80, 82, 84 orother heat transfer structures. The cavities 64, 68 and air gap 40 areinterconnected regions of the rotor cavity 60. Therefore, a heattransfer fluid added to one cavity will flow to others. The quantity ofoil or other heat transfer fluid added to cavities 64, 68 may be equalto or less than 50% of the total volume of cavities 64 and 68, equal toor less than 25% of the total volume of cavity 64 and 68, or anothersuitable volume.

When the rotor 16 spins at relatively higher speeds, the action of therotor 16 and or the heat transfer structures 78, 80 may cause splashingand misting of the oil or other heat transfer fluid, improving thethermal properties of the forced fluid circuit 74. Any retainer band 52may be fitted with a perforated coating or thin mesh surface to ensureturbulent flow and low drag as the rotor 16 moves through the oil bath.

D. Rotor Back Assembly Heat Export

As is generally described above, the rotor back assembly 36 conductsheat from the edges of the permanent magnets 34 facing the shaft axis22, and generates some heat through eddy currents and hysteresis losswithin the rotor back assembly 36 itself. The heat conducted to orgenerated within the rotor back assembly 36 may be conducted radially toventilation channels 72. Alternatively, heat conducted to or generatedwithin the rotor back assembly 36 may be conducted axially to the SEheat transfer structure 78, the OSE heat transfer structure 80, anotherheat transfer structure, or the fan 70 where heat can be conveyed to theforced fluid circuit 74. The surfaces of the rotor back assembly 36facing cavities 64, 68 may be structured, textured, anodized orotherwise treated to enhance the export of heat from the rotor 16 to theforced fluid circuit 74, stator 32, housing 12 or other structure fromwhence the heat may be dissipated into the environment.

The apparatus and methods disclosed herein, including but not limited tothe ventilation channels 72 heat transfer structures 78, 80 andthermally conductive rotor encapsulant 90 assure that the rotor backassembly 36 is relatively cooler than the permanent magnets 34 duringmotor operation. This temperature gradient causes heat flow from thepermanent magnets 34 to the rotor back assembly 36 and out of the motorthrough the shaft 14, forced fluid circuit 74 and housing 12 or anotherexport path as described herein.

E. Shaft Heat Export

Another path for transmitting heat away from the rotor utilizes theshaft 14. The shaft 14 is firmly connected to the rotor back assembly36, usually with metal-to-metal contact. Heat from the permanent magnets34, rotor back assembly 36 or other rotor structures may therefore beconducted to the shaft 14. Heat export through the shaft 14 may beenhanced by providing the shaft 14 with a relatively highlyheat-conductive core 98, or other shaft structure, made of a materialhaving relatively high thermal conductivity such as aluminum or copper,when compared to the surrounding steel shaft material. The shaft 14connects to the body of the equipment being driven, for example, a fan,pump, drive roller, or material processing machine. Thus, the shaft 14,particularly if it is provided with a heat conductive core 98, canconduct heat to the driven machine, where the heat may be dissipatedthrough convection, conduction or radiation.

Stator Cooling Methods and Apparatus

The primary sources of heat generation in the stator 32 are magneticallyinduced eddy currents within the metal core 44 of the electromagnets 38and resistance losses in the windings 42. The quantity of heat producedin a stator 32 may be reduced by maximizing the wire gauge of thewindings 42 to maximize slot fill and reduce AC resistance losses. Inaddition, rectangular wire may be utilized for the windings 42 toincrease slot fill and reduce resistance losses. Magnetically inducededdy currents within the steel electromagnet cores 44 may be minimizedby fabricating each core from electrically isolated laminations asdescribed in detail below.

Several apparatus and methods are disclosed for cooling an electricmachine stator and dissipating heat from the machine. It is important tonote that the stator 32, particularly in a TENV machine, is entirelyenclosed within the machine housing 12. Therefore, cooling the stator 32often involves heat transfer to another motor structure, for example thehousing 12, or shaft 14, prior to heat dissipation from the motor 10. Incertain instances, the stator cooling methods and apparatus describedherein operate in conjunction with methods and apparatus for coolingother portions of the motor 10, rotor 16 and/or housing 12. The variousthermal management methods and apparatus make be combined in anyfashion, scaled, or partially implemented to achieve desired thermalmanagement and machine durability goals.

A. Electromagnet Structure

As shown in FIGS. 10, 13, and 14, a representative electric machinestator 32 includes a plurality of electromagnets 38 radially positionedaround the shaft axis of the machine, for example the motor 10. As bestviewed in FIG. 14A, and the exploded view of FIG. 14B, theelectromagnets 38 include a core 44 of a magnetic metal, typically asteel alloy. Magnetically induced eddy currents can be reduced byfabricating the core 44 from a stack of similarly or identically shapedand relatively thin laminations 46. The specific lamination shape shownin FIG. 15 includes a tooth portion 100 and a yoke segment 102. Whenmultiple laminations 46 are stacked to form an electromagnet core 44 thetooth portions 100 directly or indirectly supports the windings 42 whilethe yoke segments 102 provide structure to the stator 32, a heattransfer pathway to the exterior of the stator 32, and additionalmagnetic core mass.

It is necessary to provide electrical insulation between adjacentlaminations 46 to reduce magnetically induced eddy currents in the core44. Therefore, the opposing planar surfaces of each lamination 46 may becoated with a lacquer, epoxy, plastic, insulating paint, paper, oranother dielectric layer or coating to provide electrical insulationbetween adjacent laminations 46 when multiple laminations are stacked tofabricate an electromagnet core 44. Conventional lamination insulatingmethods tend to also thermally insulate each lamination and restrict theflow of heat generated either within the core 44 or within thesurrounding windings 42.

The magnetic steel alloys used for core laminations 46 typically haverelatively low thermal conductivity. The thermal performance of anelectromagnetic core 44, and thus the thermal performance of an electricmachine, may be enhanced by coating or otherwise associating themagnetic steel laminations 46 with a material having relatively higherthermal conductivity than magnet steel. For example, some portion, orthe entirety, of the exterior surfaces of a steel lamination 46 may becoated with, plated with, have deposited upon, or otherwise beassociated with a relatively thin thermal transmission layer 104 of ametal such as nickel, nickel silver, copper, aluminum, graphene oranother material having higher thermal conductivity than the steellamination interior. An electrical insulation layer 106 may be addedover the thermal transmission layer 104 to minimize eddy currents whenthe laminations 46 are stacked into an operable configuration. Incertain embodiments one layer may serve both as an insulator and as athermal transmission layer, provided the material selected is adielectric material and has greater thermal conductivity than themagnetic steel used for laminations. For example, various grapheneoxides could serve as a single layer providing both enhanced thermaltransmission and electrical insulation.

One example of a layered lamination configuration having higher thermalconductivity than a simple insulated steel lamination is shown in FIG.15A. In this embodiment, the steel lamination 46 is coated first with athermal transmission layer 104 having higher thermal conductivity thanthe steel of the underlying lamination 46. The thermal transmissionlayer 104 may then be coated or otherwise associated with a dielectricinsulation layer 106.

Alternatively, as shown in FIG. 15B, only one planar surface of alamination 46 may be coated with or otherwise insulated with adielectric insulation layer 106. The opposing planar surface of saidlamination 46 may be coated with, contacted with, bonded to, platedwith, or otherwise associated with a thermal transmission layer 104 of amaterial selected to facilitate heat transfer away from theelectromagnetic core 44. If a core 44 is fabricated from a plurality oflaminations 46, prepared in this fashion, the thermal transmission layer104 is not required to provide electrical insulation since thedielectric insulation layer 106 of the immediately adjacent lamination46 will electrically insulate both laminations from each other, providedeach lamination is oriented in the same fashion, as shown on FIG. 15B

The thermal transmission layer 104 can be, for example, a metal havingsignificantly higher thermal conductivity than steel deposited on thelamination 46. Representative metals having higher thermal conductivitythan steel include, but are not limited to, copper, nickel, gold,silver, or aluminum. Other materials, for example graphene or graphingoxide, may be deposited as a thermal transmission layer 104 on alamination 46. A combination of the above materials may be used. Forexample, as shown on FIG. 6C, a core 44 may be fabricated with a stackof interior steel laminations 46 having a dielectric coating 106deposited or otherwise associated with one surface of a steel lamination46, which is coated on the opposite surface with a metal 104 a, forexample nickel silver, which is subsequently coated with a graphenelayer 104 b. As this pattern repeats itself through the stack, eachlamination 46 is insulated from adjacent laminations, yet eachlamination is also in contact with one or more thermally conductivetransmission layers 104 providing for heat export from the core 44.

The stacked laminations 46 forming the electromagnetic core 44 must beheld together during assembly and operation and insulated from thewindings 42. Conventional electromagnetic cores are often held togetherby glue, lacquer, screws, bolts, pins, crimped surfaces, otherfasteners, welded joints or other means. Insulation may be provided bythe glue or lacquer, a supplemental structure such as tape or paper, ormerely with the winding insulation. The disclosed embodiments could beimplemented with any one of the foregoing assembly and insulationtechniques. Alternatively, as shown in FIG. 14, the stack of laminations46, and any lamination coatings 104, 106 can be tightly compressedtogether into a core 44 having superior thermal properties with adielectric bobbin 108 surrounding the top, bottom, and sides of thetooth portion 100 of each lamination 46 in the core 44. The dielectricbobbin 108 may be fabricated, by injection molding for example, from aplastic, nylon or similar material. The utilization of a bobbin 108 as ameans of compressing the lamination faces together facilitates the useof thermal transmission layers 104 which might be compromised by the useof alternative lamination attachment methods including but not limitedto glue or lacquer between each lamination face, bolts or screws throughadjacent laminations, welded joints positioned along one or more sidesof the lamination stack, mechanical crimp connections betweenlaminations, notches or other fixturing methods.

Specifically, the bobbin 108 assures constant, evenly distributedpressure across the tooth portion 100 of each lamination 46 in anelectromagnetic core 44. Evenly distributed pressure minimizes gapsbetween laminations 46 or layers 104, 106 and therefore facilitates heattransfer from each lamination 46 to any associated thermal transmissionlayer 104. In addition, using a bobbin 108 to compress the laminations46 and associated layers 104, 106 into a core 44 avoids the possibilityof a bolt, screw, crimp, weld or other mechanical fastener causing ashort circuit between one or more laminations 46 and therebycompromising the electromagnetic properties of the core 44. In certainembodiments, the bobbin 108 is the only structure holding adjacentlaminations together.

As also shown in FIG. 14, the bobbin 108 supports the windings 42.Certain portions of the windings 42 extend through the slot betweenadjacent electromagnetic cores 44. Other portions of the winding 42 areend turns 109 bridging adjacent slots, across each electromagnetic core44.

As shown in FIG. 14, the yoke segment 102 of each lamination may beformed into opposing male and female dovetail, tongue and groove, orother mating structures, 110 and 112 respectively. Thus, when a seriesof electromagnets 38 are assembled into a stator, as best shown in FIG.13, adjacent male and female tongue 110 and groove 112 structuresprovide mechanical support to the stator 32. Furthermore, the taperedshape of each tooth 100, bobbin 108 and winding 42 create a concentratedwinding coil that tapers to the shape of the slot, providing for highslot fills. High slot fill results in a reduction in electricalresistance and hence lower losses and reduced heat production.

B. Stator/Housing Interface

The stacked, outwardly facing surfaces of a plurality of yoke segments102, defines an exterior surface 114 of the stator 32. As shown in FIG.13, the exterior stator surface 114 fits closely within the perimeterportion 54 of the housing 12. Heat can flow within each core 44 to theexterior stator surface 114, particularly if thermal transmission layers104 are provided on or between the laminations 46 as described above.The transfer of heat from the exterior stator surface 114 to theperimeter portion 54 of the housing 12, and subsequently to the outsideenvironment, may be facilitated by providing a thermally conductivelubricant between the exterior stator surface 114 and an interiorsurface 116 of the perimeter portion 54 of the housing 12.Representative thermally conductive lubricants include but are notlimited to thermal grease, graphene, or boron nitride powders.

Overall machine robustness and thermal performance is also furtherenhanced by carefully coupling the stator 32 to the housing 12. Forexample, as shown in FIGS. 12, 13 and 16, one or more of the end plates56, 58 may define a lip and shoulder structure on the end plate 56, 58that contacts the yoke segment 102. The lip/shoulder structure 120engages the yoke portion 102 of each electromagnetic core 44 somewhataway from the balance of the end plate 56, 58. Thus, the lip/shoulderstructure 120 creates a pocket in the end plate providing clearance forthe end turns 109 of windings 42. The height of the lip/shoulderstructure 120 (marked as (h) on FIG. 16), may be selected to reduce thedistance from the end turns 109 to the end plate 56, 58. facilitatingheat transfer. As described in detail below, the remaining gap may befilled with a thermally conductive encapsulant or other structure ormaterial to facilitate heat transfer. In addition, the height h of thelip/shoulder structure 120 can be increased as needed to minimize theformation of magnetically abused any currents in the end plate 56, 58.

Additional machine robustness may be enhanced by providing selectedelectromagnet cores 44 with an engagement structure 122 at, or in, theexterior surface 114. The housing 12 may include a mating engagementstructure 124 opposite the engagement structure 122. The engagementstructures 122 and 124 may be any shape or size configured tomechanically mate the exterior surface 114 of the stator 32 with theinterior surface 116 of the housing 12, and therefore prevent rotationor other movement of the stator 32 when under load. The specificembodiment of engagement structures 122, 124 illustrated in FIG. 13includes a rounded slot formed in the electromagnet core 44 and acorresponding rounded slot formed in the perimeter portion 54 of thehousing 12. Each slot may be engaged with a pin to meet the stator 32 tothe housing 12. Additional robustness is provided by tightly fitting theexterior surface 114 of the stator 32 to the interior surface 116 of thehousing 12 to harden the case and protect the electromagnets 38 fromimpact, vibration, or other forces.

C. Stator Encapsulation and Stabilization

Additional thermal transfer from portions of the stator 32 or othermachine structures and enhanced stator robustness may be provided withthermally conductive stator encapsulants, thermal transmissionstructures, or potting materials. For example, certain embodiments mayinclude a thermally conductive stator encapsulant 96 encapsulating muchof the stator structure. In the particular embodiment of FIG. 11, thethermally conductive stator encapsulant 96 encapsulates the entirety ofthe electromagnet assemblies 38, but for the exterior stator surface 114and the inside faces 126 of some or all of the magnetic cores 44 facingthe air gap. Additionally, the thermally conductive stator encapsulant96 directly contacts one or both housing end plates 56, 58 or otherhousing structure, providing a direct thermal pathway from the stator 32to the housing 12.

Alternative structures may be used in lieu of or in combination withfull or partial stator encapsulation. These alternative structures arewithin the scope of this disclosure. For example, thermal transfer fromthe stator may be provided by a separate thermal contact structure, forexample a thermally conductive, compressible or conformable solidmaterial placed into contact with some portion of the stator 32 and thehousing 12, or placed between a portion of the stator encapsulant 96 andthe housing 12. A thermal contact structure could be substantiallysolid, or have a honey-comb structure, wave washer structure, or thelike. A thermal contact structure could be fabricated of a thermallyconductive felt, foam, metal, coated metal, conformable epoxy,composite, for example a silicon based pad with alumina filler, or anyother suitable thermally conductive compound, material, or combinationof materials.

In embodiments where gaps between the stator 32 and housing 12 arepartially or substantially filled with a stator encapsulant 96, forexample, in embodiments where the stator encapsulant 96 substantiallyfills a perimeter portion of the housing 12, structural benefits beyondthermal management are provided. Encapsulation of the electromagnets 38in a stator encapsulant 96 physically prevents contamination of theelectromagnets 38 by moisture or particulate matter. Such contaminationcan cause insulation degradation and eventually cause shorts betweenadjacent wires. Furthermore, a dielectric encapsulant 96 providesincreased protection against manufacturing defects in the wireinsulation. Thus the encapsulant 96 can provide redundant protectionagainst wire-to-wire shorts, supplementing the insulation on thewindings 42. In certain embodiments, the encapsulant 96 alsoencapsulates the coil phase electrical connections, which givesadditional protection against phase-to-phase shorts and manufacturingimperfections in the wire or electrical junctions.

The encapsulant 96 also reduces wire vibration within the stator 32.Vibration can reduce the integrity and life of the windings 42.Therefore, stator encapsulant 96 provides for enhanced thermalmanagement and enhanced machine robustness. Various sensors 85,including without limitation, one or more heat and vibration sensors,may be embedded into the stator encapsulant 96 so that any increase inheat buildup or increase in stator or rotor vibration can be detected.The sensors 85 may be wired, wireless, Internet of Things (IoT), orother varieties of monitoring sensors or sensors of another appropriatetype. Monitoring electric machine operation through one or more sensorsallows for the remote detection of any deterioration in machineperformance, and for preemptive measures to be taken.

In one embodiment, the stator encapsulant 96 includes a relatively rigidexterior portion surrounding the overall stator structure and contactingthe housing as described below. As shown in FIG. 10 however, there isrelatively little room between the windings 42 of adjacentelectromagnets 38, so it can be advantageous to pot interior portions ofthe stator 32 with a relatively fluid interior potting material. Asdescribed in detail below, any type of stator encapsulant 96, having anysuitable consistency, can be treated to have enhanced thermalconductivity promoting the export of heat from the stator 32 to thehousing 12.

As noted above, the housing of FIGS. 1-2 and 8-13 includes a perimeterportion 54 surrounding the stator 32. Heat transfer from the stator 32to the housing 12 may be facilitated by physically contacting the statorencapsulant 96 with one or both of the end plates 56 and 58. In thespecific embodiment of FIG. 8, the stator encapsulant 96 contacts theentirety of an annular interior surface of each end plate 56 and 58.

The housing 12 might include a separate perimeter portion 54, a separatefirst end plate 56, and a separate second and plate 58 that are bondedtogether to form a housing 12. It is important to note however, that thehousing could be formed according to alternative methods. For example,in an alternative embodiment the perimeter portion 54 and one end plate56, 58 may be cast, machined or otherwise formed as a single part. Thus,in some embodiments, an end plate and the perimeter portion may be aunified structure. In such an embodiment, the stator encapsulant 96 maybe in direct or indirect thermal contact with an end plate region of theunified housing structure.

In the configuration illustrated in FIG. 11, the stator encapsulant 96and the inside faces 126 of certain electromagnet cores 44 define asubstantially cylindrical interior stator surface 128 facing the air gap40 with each end of the interior stator surface 128 being bounded bycentral portions of the first and second end plates 56 and 58.Collectively, the end plates and interior stator surface 128 define aclosed cylindrical rotor cavity 60. In certain embodiments, the motor 10includes minimal voids, not filled with the stator encapsulant 96between any stator structure and the housing.

Thus, the stator encapsulant 96, along with any thermally conductivelubricant in contact with the exterior surface of the stator 114 andinterior surface of the perimeter portion of the housing 116, causepotentially all surfaces of the stator 32, except for the interiorstator surface 128 at the air gap 40, to be in direct thermal contactwith one or more portions of the housing 12.

The thermal conductivity of the stator encapsulant 96 may be enhanced byincluding specific materials within the encapsulant matrix. Thesematerials may be included whether the encapsulant is a substantiallyrigid exterior encapsulant or a relatively fluid interior pottingmaterial. For example, the stator encapsulant 96 may be a dielectricmaterial applied in a liquid state to fill substantially all voidsoutside of the rotor cavity 60. In certain embodiments, material appliedas a liquid will fully or partially hardened into a more or less rigidstator encapsulant 96. Representative dielectric materials suitable forthermally enhancing a stator encapsulation material include, but are notlimited to suspended particles of boron nitride, silicon carbide,silica, aluminum oxide, aluminum, copper, another metal, another metaloxide, ceramic, graphene and the like. The thermal conductivity of theencapsulant 96 may further be enhanced if the suspended particles arespherical, have radially oriented fibers or have another shape ororientation designed to facilitate thermal conductivity.

Alternatively, in lieu of or in conjunction with the use of suspendedparticles, larger scaled structures may be associated with the statorencapsulant 96 to enhance encapsulant thermal conductivity. For example,a solid part, such as a metal or ceramic ring, with higher thermalconductivity than the encapsulant, can be embedded in the encapsulant tocreate a composite body that has higher thermal conductivity than theencapsulant by itself.

As noted above, the rotor-facing electromagnetic core faces 126 and aninside surface of the stator encapsulant 96 are exposed at the air gap40. These structures along with the end plates 56, 58 define theenclosed rotor cavity 60. Heat transported from the stator 32 to therotor cavity 60 has not yet been exported from the machine. Furthermore,the machine rotor 16 operates within the rotor cavity 60 addingadditional heat to this space. Heat may be transferred from the rotorcavity 60 through the housing end plates 56, 58 to the externalenvironment. Heat transfer from the rotor cavity 60 to the end platesmay be facilitated by fabricating or contacting the central region ofone or both end plates 56, 58 with one or more heat transfer structuresextending into the rotor cavity 60.

For example, as illustrated in FIGS. 2, 8 and 10-13, the central portionof each end plate 56, 58 may be in thermal contact with a heat transferstructure, 82 and 84 respectively. The configuration of heat transferstructures 82 and 84 are representative examples of any number of typesor configurations of heat transfer structure that can be mounted to,formed in, or otherwise thermally contacted with an end plate 56, 58. Ineach case, a heat transfer structure 82 or 84 contacts the end plate onone side and extends into the rotor cavity 60 on the opposite side.

A heat transfer structure 82 or 84 may be formed having a number ofpins, fins, combination pin/fins 86 or other structures designed toincrease surface area and promote heat transfer. The pin fins 86 extendaway from the end plate 56, 58 and into the rotor cavity 60. Theinterior surface of a heat transfer structure 82 or 84 may also beroughened to increase surface area or treated, for example with blackanodization to facilitate heat transfer. In addition, the heat transferstructure 82 or 84 may be fabricated from a material such as copper oraluminum with high thermal conductivity. The heat transfer structure 82or 84 may be bonded to the end plate 56, 58 or other housing structureusing heat transfer paste or another method facilitating effective heattransfer. Each of the heat transfer structures 82, 84 is illustrated asbeing substantially annular, however other shapes and configurations arewithin the scope of this disclosure.

Heat Export from the Housing

Thermal export from the housing 12 to the outside environment may beenhanced by providing the housing with feet 18 or another structurefacilitating heat transfer from the housing to a building floor,building wall, mounting bracket, machine part, or other externalstructure to which the motor 10 is attached. Thermal export through thefeet 18 may be enhanced by fabricating the feet from a material havinghigh thermal conductivity, for example aluminum, copper, thermallytransmissive composites and the like. In addition, the interface 130between the feet 18 and external structure may be contacted with orcoated with a material to enhance the conduction of heat energy from thefeet 18 to the external structure. For example, the interface 130 may becoated with a heat transfer paste or other material having higherthermal conductivity than the feet 18, copper for example.

Additional heat export from the housing may be facilitated by providingexterior portions of the housing 12 with fins, pins, or other heattransfer structures. Portions of the housing 12 may be roughened toincrease surface area or treated, for example with black anodization orthermally conductive paint to facilitate heat transfer. In addition, thehousing 12 may be fabricated from a material such as aluminum withrelatively high thermal conductivity.

Heat Export from the Stator Through the Shaft

As noted above, one path for transmitting heat away from the rotor 16utilizes the shaft 14. In addition, heat conducted from the stator 32 tothe housing end plates 54, 56, or other motor structures may beconducted to the shaft 14. Heat export through the shaft 14 may beenhanced by providing the shaft 14 with a thermally conductive core 98,or other shaft structure, made of a material having a relatively highthermal conductivity such as aluminum or copper. The shaft 14 connectsto the body of the equipment it is driving, for example, a fan, pump,drive roller, or material processing machine. Thus, the shaft 14,particularly if it is provided with a thermally conductive core 94, canconduct heat to the driven equipment, where the heat may be dissipatedthrough convection, conduction or radiation.

Furthermore, the shaft 14 is supported by bearings 24, 26 supported inbearing flanges 28, 30. Heat transfer from the housing 12 to the shaft14 may be facilitated by implementing portions of one or more of thebearings 24, 26 and/or bearing flanges 28, 30 with material havingrelatively high thermal conductivity, for example copper or aluminum. Inone specific embodiment, the bearings 24, 26 include bearing seals 132,134 fabricated from copper to facilitate heat transfer from the housing12 to the shaft 14. The bearing flange 28, 30 may also be fabricatedfrom copper or another material with relatively high thermaltransmissivity. In addition, the thermally conductive core 98 may extendlaterally to or near the shaft surface in the region where the shaft 14contacts bearings 24 and/or 26 or bearing seals 132, 134.

In certain embodiments, the perimeter portion of the housing 54, firstand plate 56 and second and plate 58 may be co-fabricated, weldedtogether, or otherwise fabricated to prevent entry into the housing.Therefore, the bearing flanges 28, 30, or another housing structure maysupport bearings 24, 26 implemented bearing cartridges that arereplaceable from outside the housing. Thus, the bearings 24, 26, whichare subject to accelerated mechanical wear compared to other movingparts of a motor 10, may be replaced without accessing the interiorportions of the housing 12, thus enhancing overall machine robustness.

Methods

Alternative embodiments include, but are not limited to, methods ofcooling an electric machine rotor, methods of cooling an electricmachine stator, methods of cooling an electric machine, methods offabricating an electric machine or parts of an electric machine, methodsof stabilizing an electric machine, and methods of fabricating anelectromagnet for an electric machine. Various methods will be apparentto those of skill in the art based entirely upon the apparatus disclosedherein.

Representative methods include a method of cooling a rotor 16 or coolingan electric machine 10 having a rotor 16 and a stator 32. The methodincludes causing the rotor 16 to rotate with respect to the stator 32 todrive an internal fan 70. The internal fan 70 causes a fluid, forexample air or an air and oil mixture, to be circulated in a fluidcircuit 74 between a first cavity 64 and second cavity 68 adjacent therotor 16 to cool the rotor 16. Heat transferred to the fluid circuit 74may subsequently be transferred to the machine housing 12 and thentransferred from the machine 10.

Another representative embodiment is a method of cooling a stator 32 orelectric machine 10 by placing a stator encapsulant 96 into thermalcontact with the stator 32 and a machine housing region 12, for examplea first end plate 56 or second end plate 58. The thermal conductivity ofthe stator encapsulant 96 may be enhanced by mixing an additive with theencapsulant 96 to increase thermal conductivity. Thus, heat generated inthe stator 32 may be conducted from the stator 32 through theencapsulant 96 to the housing 12.

Another representative embodiment is a method of fabricating a stator 32or electric machine 10 having a plurality of electromagnets 38 withelectromagnet cores 44. A plurality of the electromagnet cores 44 may beformed to include a stack of laminations 46 defining a tooth portion 100and a yoke segment 102. Each yoke segment may further define a tonguestructure 110 and an opposing groove structure 112. The electromagnets38 may be assembled into a stator 32 by mating the tongue structure 110and the groove structure 112 of each electromagnet 38 with thecorresponding tongue structure 110 and the groove structure 112 ofadjacent electromagnets 38. After the electromagnets are thus assembled,the stator 32 may be encapsulated with a thermally conductiveencapsulant 96.

Another representative embodiment is a method of stabilizing an electricmachine 10. The method includes stabilizing the rotor 16 with athermally conductive dielectric rotor encapsulant 90 in contact withadjacent permanent magnets 34 and stabilizing the stator 32 with athermally conductive dielectric stator encapsulant 96 in contact withadjacent electromagnets 38.

Various modifications and additions can be made to the embodimentsdiscussed without departing from the scope of the invention. Forexample, while the embodiments described above refer to particularfeatures, the scope of this invention also includes embodiments havingdifferent combination of features and embodiments that do not includeall of the above described features.

Moreover, while the procedures of the methods and processes describedherein are described in a particular order for ease of description,unless the context dictates otherwise, various procedures may bereordered, added, and/or omitted in accordance with various embodiments.Moreover, the procedures described with respect to one method or processmay be incorporated within other described methods or processes;likewise, system components described according to a particularstructural architecture and/or with respect to one system may beorganized in alternative structural architectures and/or incorporatedwithin other described systems. Hence, while various embodiments aredescribed with—or without—certain features for ease of description andto illustrate exemplary aspects of those embodiments, the variouscomponents and/or features described herein with respect to a particularembodiment can be substituted, added and/or subtracted from among otherdescribed embodiments, unless the context dictates otherwise.Consequently, although several exemplary embodiments are describedabove, it will be appreciated that the invention is intended to coverall modifications and equivalents within the scope of the followingclaims.

What is claimed is:
 1. An electric machine comprising: a shaft; a rotorback assembly surrounding a portion of the shaft; a plurality ofpermanent magnets radially positioned around a radial perimeter of therotor back assembly; a rotor fan comprising a plurality of fan bladesformed in an exterior surface of the rotor back assembly; and one ormore ventilation channels extending through the rotor back assembly. 2.The electric machine of claim 1, wherein the rotor back assemblycomprises a plurality of laminations.
 3. The electric machine of claim1, wherein at least one of the plurality of permanent magnets comprisesa plurality of permanent magnet laminations.
 4. The electric machine ofclaim 1 further comprising: a gap between adjacent permanent magnets;and a thermally conductive material comprising a polymer doped with anadditive to enhance thermal conductivity filling the gap betweenadjacent permanent magnets.
 5. The electric machine of claim 4, whereinthe additive comprises one or more of boron nitride, silicon carbide,aluminum powder, aluminum oxide powder, copper powder, and graphene. 6.The electric machine of claim 4 further comprising a heat transferstructure in thermal communication with the thermally conductivematerial.
 7. The electric machine of claim 1 further comprising: ahousing supporting the shaft; at least one shaft bearing mounted to thehousing; and a bearing seal attached to the shaft bearing, wherein thebearing seal comprises a material having greater thermal conductivitythan the shaft bearing.
 8. The electric machine of claim 7, wherein theat least one shaft bearing is removable from an exterior surface of thehousing without opening the housing.
 9. The electric machine of claim 7,wherein the housing comprises one or more thermally conductive feetattached to the exterior surface.
 10. The electric machine of claim 9wherein the thermally conductive feet comprise a separate heat transfermaterial positioned between the thermally conductive feet and a mountingsurface.
 11. The electric machine of claim 7, wherein the housingdefines a totally enclosed non-ventilated (TENV) housing.
 12. A methodof cooling an electric machine comprising: providing the electricmachine comprising; a shaft; a rotor back assembly surrounding a portionof the shaft; and a plurality of permanent magnets radially positionedaround a radial perimeter of the rotor back assembly; a rotor fancomprising a plurality of fan blades formed in an exterior surface ofthe rotor back assembly; one or more ventilation channels extendingthrough the rotor back assembly; conducting heat from the permanentmagnets to the rotor back assembly; and exporting heat from the rotorback assembly by causing a fluid to circulate through the one or moreventilation channels with the rotor fan.
 13. The method of claim 12further comprising: providing a housing surrounding the rotor backassembly; and exporting heat from the rotor back assembly to the housingby contacting the fluid with the housing.
 14. The method of claim 13further comprising: providing one or more heat transfer structures inthermal contact with at least one of the rotor back assembly and theplurality of permanent magnets; and exporting heat from the rotor backassembly by causing the one or more heat transfer structures to contactthe fluid.
 15. The method of claim 12 further comprising: providing ahousing supporting the shaft; providing at least one shaft bearingmounted to the housing; providing a bearing seal attached to the shaftbearing, wherein the bearing seal comprises a material having greaterthermal conductivity than the shaft bearing; and conducting heat fromthe shaft to the housing through the shaft bearing seal.
 16. The methodof claim 15 further comprising removing the shaft bearing from anexterior surface of the housing without opening the housing.
 17. Themethod of claim 15 further comprising: providing the housing with one ormore thermally conductive feet; and exporting heat from the housing to amounting surface through the one or more thermally conductive feet. 18.The method of claim 17 further comprising positioning a thermallyconductive material between the thermally conductive feet and a mountingsurface.